1. Field of the Invention
The present invention generally relates to the field of blow molding articles from plastic parisons, and more particularly concerns methods and systems for producing stretch blow molded articles formed of polymeric resin and having consistent physical dimensions.
2. Description of the Prior Art
Providing consistent physical dimensions, including consistent material distribution, has been a chronic problem in production of blow molded articles, such as those articles formed from polymeric resins such as polyethylene terephthalate (PET). Difficulties in achieving consistent physical dimensions, such as material distribution, has led to problems in assuring acceptably high quality of such articles. In addition, difficulties arising from the lack of uniform physical dimensions can exist across various production platforms and between production facilities seeking to produce substantially identical blow molded articles. Consistent physical dimensions such as material distribution are very important for end performance characteristics of blow molded articles, e.g., stretch blow molded containers, such as burst strength, top load strength, thermal deformation resistance, and stress cracking resistance.
Current material distribution analysis techniques directed to already formed blow molded articles are time consuming, relatively crude in that such techniques provide only gross material distributions, and, in addition, do not lend themselves to automated inspection. One current analysis technique for material distribution is the so-called section weighing method, where, for instance, portions of a stretch blow molded article such as a base, label panel, and shoulder, are cut away from each other and individually weighed to ascertain whether these sections weigh within prescribe tolerances. Despite the availability of the section weighing method, and while this method is adequate for gross determination of overall material distribution of the sections, the section weighing method cannot account for variability within the section, although variability within a section of a blow molded article has been frequently observed in both laboratory and production environments. Further, the section weighing method is necessarily destructive of the article and can only be performed on a very small number of the articles, while most (perhaps as much as 99.9%) of the articles manufactured by a particular line are not inspected at all for material distribution. Therefore, the section weighing method can only be used to indirectly determine whether the material distribution of a stretch blow molded article which has not been cut and weighed but was made using the production parameters of a section weighed article is within acceptable tolerances. Further, there is no easy method of integration of the results of such testing back into the manufacturing process to achieve articles having more uniform or desirable parameters.
Consequently, there exists a need in the art for methods and systems for directly and non-destructively assuring consistent physical dimensions such as material distribution for stretch blow molded articles, in order to produce stretch blow molded articles of consistent quality across various production platforms and between different production facilities. Preferably such systems can be applied to a substantial portion, if not all, of the containers manufactured in a given production line so that product quality can be maintained to better standards.
Our prior patent, namely, U.S. Pat. No. 5,902,526, disclosed a method for inspecting blow molded articles on a continuous or substantially continuous basis, wherein the parisons employed in the blow molding process included a plurality of first markers formed at preselected position on the parison exterior surface. The first markers were generally in the form of light circumferential lines disposed generally planarly and parallel to each other, each line forming a complete outwardly projecting annulus on the exterior surface of the parison. The product is formed by placing such a parison in a blow mold having sections defined generally by smooth interior surfaces, but having a plurality of second differentially dimensioned portions, each second differentially dimensioned portion disposed at a predetermined location on stretch blow mold interior surface. The location of each second differentially dimensioned portion was generally selected to correspond with the optimum location of one of the first markers when the parison was transformed by the blow molding process to conform to the interior of the blow mold. The second differentially dimensioned portions could be disposed on the blow mold interior surface in any of a variety of configurations and orientations, such as where the individual second differentially dimensioned portions are formed of small or large differentially dimensioned segments disposed planarly or non-planarly, or as raised or depressed impressions on the blow mold interior surface, or by modified surface finish of blow mold interior surface.
In a finished article formed in accordance with U.S. Pat. No. 5,902,526, the relative position of first markers with respect to the corresponding, proximately disposed second markers indicates whether at least one dimension is within the preselected range. For example, the thickness dimension of a profile of the article may be determined to be within a preselected range of thicknesses by ascertaining whether the distance between each first marker and a corresponding second marker is less than a preselected distance. The marker can be formed to define tolerance bands or zones. Upon blowing the parison with the first markers within such a mold produces an article having second markers defining a tolerance zone or band, and including a plurality of first markers in close relation to the plurality of corresponding tolerance zones. The relative position of the first markers with respect to corresponding, proximate tolerance zones indicates whether at least one dimension is within a preselected range. Where the first markers lie within corresponding tolerance zones, the at least one dimension will be within a preselected range. However, if a first marker lies outside the corresponding tolerance, then the at least one dimension is not within a preselected range.
Inherent in the process of U.S. Pat. No. 5,902,526 is the need to form both the first marker on the parison and the second marker upon blow molding the finished article, followed by comparing their relative location. While such overlapping or closely proximate markings can be formed in a manner to be detected by individual visual inspection, the use of such a system of markers in connection with any automatic inspection system has proven to be elusive if not impossible. U.S. Pat. No. 4,131,666 also employed a surface grid marking technique followed by visual examination to determine the distribution of plastic in a finished container, but did not suggest any manner of converting the visual inspection to one that might be automated.
The control of variations in dimensions of finished articles through modifications in process parameters is taught generally by U.S. Pat. Nos. 3,934,743 and 4,044,086. Other patents, e.g., U.S. Pat. Nos. 3,956,441; 4,307,137; and 4,564,497, have disclosed structures included on the surface of parisons to achieve decorative effects on the finished articles. Still other patents, e.g., U.S. Pat. Nos. 4,151,249; 4,320,083; 4,359,165; 4,785,950; 4,927,679; 4,997,692; 5,101,990; 5,116,565 and 5,312,572, have disclosed structures included on the surface of parisons to achieve structural effects in the finished articles. U.S. Pat. No. 4,117,050 discloses the longitudinal thermal profiling of a parison to control wall thickness distribution in blow molded articles, but does not discuss any scheme for automated inspection of the resulting articles. U.S. Pat. Nos. 4,571,173 and 5,066,222 disclose schemes by which the temperature of a parison is heated in a non-uniform manner as a function of time to achieve the optimum heat profile for blow molding, but again there is no discussion of any scheme for automated inspection of the resulting articles.
There continues to be a need for a control system that can be employed in a continuous production process, wherein the system relies at least in part on some direct or indirect measurement of one or more selected criteria of the finished blow molded articles, to control or modify the manufacturing conditions so that the material distribution for a sequence of blow molded articles, such as stretch blow molded containers, remains within some defined range of normal values.
In order to aid in the understanding of the present invention, it can be stated in essentially summary form that it is directed to a method and system for the production of blow molded articles, such as stretch blow molded containers, from injection molded parisons, having well controlled physical dimensions so that optimum articles are produced consistently. The method and systems of the present invention include non-destructive, direct measurement of the molded articles to ensure consistent and controlled physical dimensions for such molded articles, providing an accuracy level for physical dimensions and resultant quality of such molded articles that far exceeds that available from current methods and apparatus.
More specifically, the present invention involves a method for inspecting the material distribution of a sequence of stretch blow molded containers in a continuous production process, by providing injection molded parisons, each parison having at least one indicator for detecting material distribution at a predetermined position along the axial extent of the parison. Each of the parisons is subjected to a heating stage in the continuous production process, generally by exposure to infrared heating. Each heated parison is then subjected to a blow molding process, which can be a stretch blow molding process, so that each material distribution indicator is transformed to a corresponding position along the axial extent of the blow molded container or other article. A detector is positioned to detect the location of each indicator on each blow molded container from a given mold in the production process. A computation is then made to determine for each blow molded container a material distribution outcome or profile based on the detected locations of each indicator. This information on material distribution is them matched to a prescribed profile for the particular container design and, if any discrepancy beyond some preset tolerance value is detected, a mechanism is employed to reject the out of tolerance item from the production line. Further, a modification is made to a step of the manufacturing process based on the nature of the discrepancy such as the infrared heating pattern employed.
When the inspection method of the present invention is employed in a manufacturing process using a plurality of blow molds in a continuously repeated cycle, there is a need to compare the information on material distribution taken from articles produced in the same blow mold so that variations between blow mold are screened from the data. This is achieved by providing a scribe in a selected blow mold of the plurality of blow molds capable of molding a mark on each respective container blow molded in the selected blow. The mark identifying the marked container can then be detected and each subsequent unmarked container in the continuous cycle, if produced in serial fashion, can be related to other specific blow molds used in the process. The material distribution data for each of the marked containers can be correlated, recorded, and/or employed to modify a step of the manufacturing process such as the infrared heating pattern. The variation in the determined material distribution data for any of the subsequent unmarked blow molded containers in the cycle can be employed to address a specific problem associated with the specific blow mold in which the container was made.
In some processes, it may be unnecessary or inappropriate to adjust or modify the manufacturing process in response to the material distribution data for each of the marked containers. Instead, samples from the sequence of stretch blow molded containers can be selected on any desired basis or frequency, and the sample containers employed to calculate an average material distribution outcome for the samples based on the determined material distribution outcome for each blow molded container in the sample. This average material distribution outcome can then be compared to a standard material distribution representing a nominal blow molded container to obtain an average material distribution variance to provide a signal based on the comparison back to the manufacturing process.
The inspection process must be automated to provide the desired continuous feedback to the manufacturing process. The inspection process is most desirably accomplished by transmitting light through the molded article in the vicinity of the expected location for each material distribution indicator. A plurality of sensors is provided that are capable of sensing light transmitted through the article, with each sensor capable of changing states upon receiving such transmitted light showing the location of the material distribution indicators. A lens or lens system can be employed to capture portions of the transmitted light which is then focused on a sensor such as a line linear array reader or charged coupled device camera. An output from the array reader or CCD camera can then be employed to develop a feedback signal for use by the manufacturing control system to control one or more steps of the manufacturing process.
The present invention thus provides for the production of stretch blow molded articles with non-destructively and directly controlled dimensions, such as material thickness distribution throughout the profile of stretch blow molded articles, and for consistent quality control across platforms and production facilities, ensuring that an optimum article is consistently produced with desirable performance characteristics such as burst strength, deformation, and stress cracking.
The method of the present invention for producing a stretch blow molded article having at least one dimension within a preselected range includes the step of injection molding the parison so that a parison exterior surface includes at least one indicator, typically a protruding circumferential ring or ridge, each indicator disposed at a preselected position on the parison exterior surface and formed by contact with a differentially dimensioned portion, typically a groove or indention, disposed at a preselected location on a generally smooth injection mold cavity interior surface. Where the method is carried out with a series of blow molds, the method of the present invention can include the step of blow molding an article from the parison so that an exterior surface of one article of the series includes at least one mark to identify and/or distinguish the output of one of the series of blow molds from the output of the remaining molds of the series so that articles from the same mold or station can be compared to each other and/or to an established standard.
The method of the present invention also includes inspecting the molded articles to determine whether the at least one indicator is positioned within a tolerance zone. The inspecting step is performed by situating at least one source of light and at least one positionally discriminating optical receiver, such as a CCD camera, adjacent to the output of the series of molds so that the light from the source projects through the molded container or other article and on to the optical receiver. Each optical receiver determines the position of an indicator on each passing blow molded article and provides an output signal indicative of the indicator position. The output signals from the optical receivers are then processed to compute the corresponding material distribution of each molded article from which the data was derived. The computed material distribution information can then be employed to control a subsequent step of the manufacturing process, such as an individual molded article rejection or ejection step, and/or a prior step of the manufacturing process, such as radiant heating control to modify the thermal profile of the parisons prior to the blow molding process.
It is an object of the present invention to provide an apparatus for producing a blow molded article having at least one consistent and controlled dimension. This object is achieved by providing an apparatus for producing a blow molded articles utilizing direct, non-destructive analysis of the stretch blow molded article to control at least one manufacturing variable. This control of at least a portion of the manufacturing process through a substantially continuous analysis of one or more indicators molded on the surface of the molded article has the advantage of permitting a closer and more responsive control of the manufacturing process through non-human intervention means, which allows for greater reliability than previously possible.